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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

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FUELS (GENERAL METHODS)

304

tube carrying a manometer and the bomb slowly filled with oxygen * until
the pressure is 20-25 atmos. (or less, if the coal burns very easily). The
bomb is then placed in the calorimeter ,4, into which the proper amount
of water is poured, the thermometer t being placed in position, the
stirrer d started and the temperature read off every minute. After five
minutes, ignition is caused by the momentary passage of the current. The
temperature is read half a minute after ignition, after a further half-minute,
and then each minute until the maximum temperature is reached (after
3 or 4 minutes) and for five minutes during the subsequent fall in tem-
perature.

At the end of the experiment, the tap r of the bomb is opened to allow
the gas to escape, the bomb itself being then opened and washed out inside

with   a   little   distilled    water.2
The nitric acid formed from the
nitrogen contained in the bomb
is determined in  the wash water
by titration with caustic   potash
solution    (i    c.c. = o-oi    gram
HN03)   in   presence   of   methyl
orange.       Any    sulphuric    acid
formed    is    also    calculated   as
nitric acid, but  with fuels poor
in sulphur no appreciable error is
introduced in . this way.    When,
however, allowance is to be made
for the sulphuric acid, the proce-
dure is as follows:    The  wash
water is heated for a short time
to   expel   carbon    dioxide    and
titrated   with   N/io-baryta    in
presence    of     phenolphthalein;
excess of  standard sodium car-

FlG                                    bonate solution is then added and

the   excess titrated  with N/io-

hydrochloric acid in presence of methyl orange. The volume of baryta
solution used corresponds with the sulphuric and nitric acids together,
and that of the sodium carbonate solution with the nitric acid alone.

In calculating the results of the measurement, it is first necessary to
establish the correction necessary owing to the exchange of heat with the
surrounding air in the interval of time between ignition and the attainment

1  Compressed oxygen, if obtained electrolytically, often contains hydrogen, which
appreciably alters the results of the calorimetric experiments.    In such case it is neces-
sary to purify it, before admitting it to the bomb, by passing it slowly through a red-
hot copper tube and then through a coil cooled with water.    On the other hand, oxygen
from liquid air, containing appreciable quantities of nitrogen, has the disadvantage
of giving rise to the formation of nitric acid, allowance for which must be made in
calculating the results.

2  With fuels poor in hydrogen and hence yielding little water when they burn, a
few c.c. of water may be placed in the bottom of the bomb before closing it so that
the products of oxidation of the nitrogen and sulphur may be dissolved,

ter t divided